Design problem with a constant torque motor

  • Thread starter pete
  • Start date
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Summary
Run into a variation on the output of this constant torque spring assemble and am looking for advice on the design to remove this.
I’ve been trying to design a counterbalance system using a constant torque motor spring assembly. The spring I’m looking at has 25.4kg cm torque on the shaft from the output drum.

So if I put a 100mm diameter cable drum on this shaft I should be able to counterbalance 5kg over the given extension of the spring. I’m unclear on how to calculate the exact amount of revolutions of the output shaft but it should be in the ballpark of about 21. Let's say I use a 1mm diameter Dyneema cable. I can go smaller but I’d like to have the option to use pulleys and smaller cable drums to increase the weight the system can hold and also I’d worry about a thinner cable cutting into the drums.

As the cable winds onto the drum it will stack on top of its self and a change of even 2mm, 100mm to 102mm, diameter on the drum will be about a 100g change in the counterbalance. The carriage supported is on rails using bearings so the friction is low and I worry that this will be enough to put the whole thing out of balance.

I spent some time researching counterbalance design using these spring motors but there's not much out there so I thought I come here and see first of all if I’ve got this right and second if there is some solution someone can see that I’m just not thinking of.

This is the spring: https://www.ondrives.com/sr116

This is an example of an assembly: https://www.ondrives.com/ba274-s

Any advice appreciated.
 

JBA

1,338
335
Are you asking about suggestions for a different type of counterbalance system; or a modification to eliminate the force differential problem with the current constant torque motor.
Note: Based upon the 25 revolution limit of your motor, if you use a multiple pulley arrangement you will need to increase the cable drum size in order to maintain your current travel distance and that will reduce the pulling load of the motor so you will end up with no force gain for your effort.
 
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Yes, I'd have to lose travel to gain force but that's something that I'd like to be able to do.

I'm looking for a solution to the problem of the varying force due to the cables building up on the drum or another counterbalance option I guess but I'm not sure what else would have the travel or adaptability.
 

Tom.G

Science Advisor
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How about just adding some friction to mask the torque variation and use a slightly stronger spring or different size drum?
 
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Adding friction would work but I really want to keep the friction low, it's important it moves freely.

It's relative to the drum size, as the diameter increases the force for any variation on the diameter decreases. Stronger springs help but these are expensive and heavy, you'd improve the situation by tripling the weight and cost but not solve it.

Using pulleys is the best I can think of, this will increase the load/diameter at the sacrifice of travel and I can lose travel.

It just feels like bad design. The problem is never fixed just improved. I've been trying to come up with a way to implement some kind of storage drum feeding cable to the drum on the output shaft but have come up with nothing that could work.
 

JBA

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What is the source of the load you are trying to counterbalance i.e. is it the weight of a vertical load, a load traveling on a sloped surface or a mechanical load of some type?
 

Tom.G

Science Advisor
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Two other possibilities come to mind, but they increase size and/or complexity.
1) Make the take-up drum long enough to wind the cable in a single layer
2) Use two drums and motors. The drum closest to the load will do most of the work and have a few turns of cable around it, and the 'free' (retracted) end of the cable will be taken up by a second drum.

In approach 2),
If the cable slips on the first drum, either add more turns around it or increase the tension from the second drum.
If side-to-side wandering of cable on the first drum is a problem, try giving it a slight hourglass shape... or adding an ordinary pulley downstream of it.

Cheers,
Tom
 
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I will have to make something up and test it to see exactly what the problem looks like, maybe it will lay down in a neat row every time naturally, but I doubt it.

The diameter of the take-up drum dictates the counterbalance load so is fixed.

If the cable moves from one side to the other without passing over its self then at a width of just over 20mm its possible but how to feed the cable onto the drum in such a way it always does this?

I spent some time looking at drum profiles and idlers and such but all the ideas I got down either looked unconvincing or became too complex. I can find no standard solutions for this, only in much larger cable drums where a dedicated mechanized guide idler is used.

My first solution is to put an idler between the load and the drum. I'll put some thread on its shaft with a pitch of 1mm then connect it to the output shaft from the drum so they both turn 1:1 and the threaded connection should then move the idler 1mm per turn along its shaft. I need to draw this up but it looks promising.

The second option is a second drum. A storage drum as you say with its own small spring to keep the cable under tension passing a few loops around the output drum to set the load. But Dyneema is slippy stuff and I'd worry that by the time you'd made enough loops to make it secure you'd be back to the same problem, though admittedly in a much-reduced way. As the output drums are changeable to change the counterbalance load it would also make changing the drums very fiddley.

However, If I replaced the cable with a timing belt. I'll have to look it up but I'm pretty sure something like a HTD 5M 9mm wide belt can happily carry the 20kg max load, then your idea of the second storage drum could feed changeable pulleys. Never slips, no backlash and off the shelf parts. I could get the needed 8 meters onto something like a 170mm diameter drum.

I think both could be workable solutions.
 

JBA

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While it is more complex, one solution to control the wrapping for long single wrap drum would be to use the type of line guide that is used to control line wrapping on fishing reels. You have a fixed reciprocating system with a single wrap on the drum, so you could simplify that mechanism by using a screw thread on a free turning shaft that receives and feeds the line to your power drum. It could have one initiating wrap and tension from your load on the line would keep it in the groove.
 

Tom.G

Science Advisor
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To get a cable to take-up in a single layer, the key is the approach angle of the cable to the drum. For instance if the cable has a fixed guide/constraint close to the drum, it will readily wrap over itself. Move the guide further from the drum and it tends to wrap in a single layer.

I've never bothered any detailed calculations but I expect controlling variables to be:
  • Approach angle
  • Cable diameter
  • Cable stiffness
  • Cable coefficient of friction (to itself)
  • Drum diameter

Cheers,
Tom
 

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